Juventus is already on the 4th position, what they need is to be consistent for them to maintain the fourth position till the end of the season. If they fail to improve their performance in order to be consistent, they will find themselves in UEL by next season and Lazio will replace them in UCL. I don't know what's the problem with Juventus this season.
Since Juventus are out of the Champions League race, they will focus more on the Serie A games so they have no excuse if they don't qualify for the champions league next season by making sure they remain in the top four positions. Lazio is still vibrant in the Europa League since they have a better performance there, they will apply more of their energy in the Europa League matches which will distract them from the league games, so it means that they can't do better than Juventus in the Serie A. Juventus has made improvements in the Serie A after they got many draws in their matches towards the start of the season, but they are better now and if they maintain their performance for a long time they will surely finish in the top four and qualify for the Champions League next season.